Communication system for transmitting sync-flags and pilot symbols and method thereof

ABSTRACT

The present invention relates to a communication system and method for transmitting sync flags and pilot symbols during sync symbol periods. Example embodiments provide a method for sharing sync symbols to communicate flag sequences and pilot sequences during a sequence of sync symbol periods. The method includes modulating the pilot sequence by multiplying a pilot code by each value of the pilot sequence, and modulating the flag sequence by multiplying a flag code by each value of the flag sequence. The pilot and flag codes are mutually orthogonal to each other and each includes at least two non-zero values. The method further includes generating a resulting sync symbol sequence based on the modulated pilot sequence and modulated flag sequence, and sending the resulting sync symbol sequence during the sync symbol period.

BACKGROUND

In the legacy Very High Speed Digital Subscriber Line 2 (“VDSL2”)standard, sync symbols occur periodically after every 256 data symbols.On a particular legacy VDSL2 communication line, one or more tones (i.e.subcarriers) transmit a sync symbol including one of two values during async symbol period. Typically all tones transmit the same sync symbolvalue in a given sync symbol period (when defining the value to be thevalue before any quadrant scrambling). The values may be real orcomplex. Real values may be represented by “1” or “−1.” The complexvalues may be may be 00 corresponding to “1+j” or 11 corresponding to“−1−j”. Normally, the value transmitted on all tones is the same fromone sync symbol to the next sync symbol. For example, a central officemay transmit downstream the value “1” repeatedly on all active tones.When the central office receives an online reconfiguration request (OLR)from a customer-side equipment, the central office may then switch thevalue to “−1” and transmit “−1” repeatedly to the customer-sideequipment. Such a transition is interpreted by the customer-sideequipment as an acknowledgment of the OLR. Such an acknowledgment isreferred to hereinafter as a sync-flag. A sequence of “1” values and“−1” values that is used to convey sync-flags (or the absence of a syncflag) will be referred to hereinafter as a flag sequence.

In the emerging G.vector amendment to the VDSL2 standard, the currentconsensus is that pilot signals will be transmitted on sync symbols. Forexample, pilot sequences may be sent instead of flag sequencesdownstream on sync symbols during the sync symbol periods to estimatecrosstalk on communication lines. For instance, a pilot sequence may beassigned to each communication line of the communication system and thepilot sequences may be sent downstream to the customer-side equipment.At the customer-side equipment, error samples are determined and fedback to the central office. At the central office, the error samples arecorrelated with the pilot sequences in order to obtain estimates for allof the crosstalk coefficients. The process of obtaining error samplesmay be repeated as needed to obtain a more accurate crosstalk estimate.These estimates will then be used to cancel crosstalk using precoding.Each symbol of the pilot sequence may be similar in nature to the legacysync symbol. For instance, each symbol of the pilot sequence may alsoinclude the real or complex values of the legacy sync symbols.

A conventional method for transmitting both the pilot sequences and flagsequences on sync symbols consists of time-sharing the sync symbols. Forexample, odd-numbered sync symbols are used to send arbitrary pilotsequences (e.g., consisting of “1”s and “−1”s). Even-numbered syncsymbols are used to communicate flag sequences. That is, the sync-flagis conveyed whenever an even-numbered sync symbol is opposite in signfrom the previous even-numbered sync symbol. As a result oftime-multiplexing the pilot sequences and flag sequences, sync-flags canonly be sent half as often as compared to a system that sends flagsequences on a separate communication line. For example, usingtime-multiplexing, sync-flags can only be sent after every 513 DiscreteMulti-tone (DMT) symbols. Assuming the symbol rate is 4 kHz, the amountof time may be increased by up to 64 ms relative to a system sendingonly flag sequences.

FIG. 1 illustrates a conventional time division scheme to transmit pilotsequences and flag sequences on the sync symbols. Referring to FIG. 1,the center row of the figure, labeled “sync symbols”, illustrates acomplex constellation point sent on eight consecutive sync symbols. Theodd symbols are interpreted as pilot values, and are used to transmitthe pilot sequence (+1,−1,−1,+1). The even symbols are used to conveyfour flag values (e.g., labeled “sync bits” in FIG. 1), and are used totransmit the flag sequence (+1, +1, +1, −1). A sync-flag is indicated bythe signal change from the 6^(th) sync symbol value to the 8^(th) syncsymbol value, or equivalently from the 3^(rd) flag value to the 4^(th)flag value.

The main shortcoming of the time division scheme is evident when thecustomer-side equipment includes a legacy modem and/or when acommunication line is a legacy VDSL2 communication line. A communicationline having a legacy modem on the customer-side equipment is hereinafterreferred to as a legacy communication line. A communication line havinga modem on the customer-side equipment adhering to the G.vectoramendment is hereinafter referred to as a G.vector communication line.

When legacy communication lines and G.vector communication lines areboth present in a communication system, the sync symbols on the legacycommunication line, which conveys only flag values, may interfere withthe crosstalk estimation being performed by the G.vector communicationlines. For example, if a communication system has four G.vectorcommunication lines, the pilot sequences may be (1,1,1,1), (1,1,−1,−1),(1,−1,1,−1) and (1,−1,−1,1). Each pilot sequence is assigned to each ofthe four G.vector communication lines, respectively, and transmittedrepeatedly on the sync symbols during the sync period. Then to estimatethe crosstalk from the second communication line into the thirdcommunication line, a sequence of four error samples measured at thecustomer-side equipment of the third communication line would becorrelated with the pilot sequence of (1,1,−1,−1) assigned to the secondG.vector communication line. Crosstalk from the other G.vectorcommunication lines would not affect this measurement because of theorthogonality of the pilot sequences.

If the same communication system includes a legacy communication line,the central office would send sync symbols on the legacy communicationline by sending flag value “1” repeatedly on all active tones until thecentral office received an OLR from the customer-side equipment, atwhich point the central office would acknowledge receipt of the OLR bysending a sync-flag by starting to send the flag value “−1” repeatedlyon all active tones. If the resulting flag sequence sent on the legacycommunication line happened to be (1,1,−1,−1), coinciding with the pilotsequence sent on the second G.vector communication line, then thecrosstalk estimate from the second G.vector communication line into thethird G.vector communication line would be corrupted by crosstalk fromthe legacy communication line.

As a result, the flag sequence on a legacy communication line maycoincide with or be strongly correlated with the pilot sequence sent onone of the G.vector communication lines. Therefore, the crosstalkestimate of the G.vector communication lines may be contaminated by thecrosstalk from the legacy communication line. For example, if acrosstalk coefficient from the G.vector communication line is g₁ and acrosstalk coefficient from the legacy communication line is g₂, a meanvalue of the contaminated crosstalk estimate is g₁+ρg₂, where ρ is thecorrelation coefficient between the flag sequence sent on the legacycommunication line and the pilot sequence of G.vector communicationline, assuming the flag and pilot sequence are transmitted at equalpower. Depending on the conditions of the communication system, ρ may beas large as 1. As a result, the error percentage may be as much as 100%error when g₁ and g₂ have similar magnitudes.

SUMMARY

The present invention relates to a communication system and method fortransmitting flag sequences and pilot sequences during sync symbolperiods. In particular, the system and method prevents flag sequencesfrom interfering with crosstalk estimation among the G.vectorcommunication lines, and prevents pilot sequences from interfering withdetection of sync-flags.

Example embodiments provide a method for sharing sync symbols tocommunicate flag sequences and pilot sequences during a sequence of syncsymbol periods. The method includes modulating the pilot sequence bymultiplying a pilot code by each value of the pilot sequence, andmodulating the flag sequence by multiplying a flag code by each value ofthe flag sequence. The pilot and flag codes are mutually orthogonal toeach other and each includes at least two non-zero values. The methodfurther includes generating a resulting sync symbol sequence based onthe modulated pilot sequence and modulated flag sequence, and sendingthe resulting sync symbol sequence during the sequence of the syncsymbol periods.

The method may further include transforming each sync symbol of theresulting sync symbol sequence into a signal for transmission across acommunication line. The resulting sync symbol sequence includes addingthe modulated pilot sequence and the modulated flag sequence. The valuesof the flag and pilot codes may be complex numbers. Each value of thepilot sequence and the flag sequence may be one of two real or complexvalues.

The method may further include allocating sync symbol power between thepilot sequence and the flag sequence by adjusting a first and secondparameters of the pilot and flag codes. According to exampleembodiments, the flag code is (a, a) and the pilot code is (b, −b),wherein

${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$wherein α²+β²=1, and wherein α and β are the first and second parametersof the flag and pilot codes. The pilot sequence may be used forcrosstalk estimation and the flag sequence may be used to send syncflags.

According to another example embodiment, a method for sharing syncsymbols to receive flag sequences and pilot sequences during a syncsymbol period may include receiving a resulting sync symbol sequenceduring the sync symbol period. The resulting sync symbol sequenceincludes the flag sequence and pilot sequence. The method may furtherinclude recovering the pilot sequence by correlating the resulting syncsymbol sequence with a pilot code, and recovering the flag sequence bycorrelating the resulting sync symbol sequence with a flag code. Thepilot and flag codes are mutually orthogonal to each other and mayinclude at least two non-zero values.

The flag sequence may be recovered by multiplying elements of theresulting sync symbol sequence by corresponding elements of the flagcode and summing the products to generate a value of the recovered flagsequence. The pilot sequence may be recovered by multiplying elements ofthe resulting sync symbol sequence by corresponding elements of thepilot code and summing the products to generate a value of the recoveredpilot sequence.

According to another example embodiment, a communication system forsharing sync symbols to communicate flag sequences and pilot sequencesmay include a first modulator configured to modulate the pilot sequenceby multiplying a pilot code by each value of the pilot sequence, asecond code modulator configured to modulate the flag sequence bymultiplying a flag code by each value of the flag sequence, and acombiner configured to combine the modulated pilot sequence with themodulated flag sequence to generate a resulting sync symbol sequence.The pilot and flag codes may include at least two non-zero values. Thepilot and flag codes are mutually orthogonal to each other.

The communication system may further include a channel modulatorconfigured to send the resulting sync symbol sequence by transformingeach sync symbol of the resulting sync symbol sequence into a channelsignal for transmission across a communication line. For example, in aVDSL2 system, one component of the channel modulator would take theinverse Fourier transform of a vector specifying the sync symbol valueassigned to each tone, in order to produce a vector of time domainsamples. The communication system is configured to allocate sync symbolpower between the pilot sequence and the flag sequence by adjusting afirst and second parameter of the pilot and flag codes.

According to another example embodiment, a communication system forsharing sync symbols to receive flag sequences and pilot sequences mayinclude a channel demodulator configured to receive a resulting sequenceof channel signals, each of which is typically affected by noise andinterference. The channel demodulator takes each of the noisy receivedchannel signals and produces a corresponding noisy sync symbol, usingchannel demodulation techniques well-known to those skilled in the art.The resulting sequence of noisy sync symbols, is processed by a firstcode demodulator configured to recover the pilot sequence by correlatingthe resulting sync symbol sequence with a pilot code and produce a noisyversion of the pilot sequence, and a second code demodulator configuredto recover the flag sequence by correlating the resulting sync symbolsequence with a flag code.

The communication system may further include a first estimation systemconfigured to receive the noisy version of the pilot sequence andgenerate an estimated pilot sequence based on one of two possible pilotvalues for the values of the pilot sequence, and a second estimationsystem configured to receive the noisy version of the flag sequence andgenerate an estimated flag sequence based on one of two possible flagvalues for the values of the flag sequence.

The communication system may further include a combiner configured tocombine the estimated pilot sequence from the noisy version of the pilotsequence to generate a pilot error sequence, and an error feedback unitconfigured to transmit the pilot error sequence to be used in crosstalkestimation.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will become more fully understood from the detaileddescription given herein below and the accompanying drawings, whereinlike elements are represented by like reference numerals, which aregiven by way of illustration only and thus are not limiting of thepresent invention, and wherein:

FIG. 1 illustrates a conventional time division scheme to transmit pilotsignals and sync symbols;

FIG. 2 illustrates a communication system to communicate flag sequencesand pilot sequences during a sync symbol period according to exampleembodiments;

FIG. 3 illustrates a modulation system according to example embodiments;

FIG. 4 illustrates a demodulation system according to exampleembodiments;

FIG. 5 illustrates a diagram depicting the transmitted resulting syncsymbol sequence, the recovered pilot sequence and recovered flagsequence according to example embodiments; and

FIG. 6 illustrates the four different complex sync symbols according toexample embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. In thedrawings, the thicknesses of layers and regions are exaggerated forclarity.

Detailed illustrative embodiments of the present invention are disclosedherein. However, specific structural and functional details disclosedherein are merely representative for purposes of describing exampleembodiments of the present invention. This invention may, however, maybe embodied in many alternate forms and should not be construed aslimited to only the embodiments set forth herein.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention. Like numbers refer to like elementsthroughout the description of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments of thepresent invention. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments of the invention. As used herein, the singular forms “a”,“an” and “the” are intended to include the plural forms as well, unlessthe context clearly indicates otherwise. It will be further understoodthat the terms “comprises”, “comprising,”, “includes” and/or“including”, when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

FIG. 2 illustrates a communication system to communicate flag sequencesand pilot sequences during a sync symbol period according to exampleembodiments. The communication system includes a central office (CO)210, a vectoring group 220 including M communication lines, and aplurality of M customer-side equipment (CPEs) 230, with one CPEterminating each communication line.

The CO 210 may include a plurality of operator-side modems, eachconnected to one of the M communication lines, and a vectoring controlentity (VCE) which coordinates the transmissions on the communicationlines. For example, the vectoring control entity may be configured tocoordinate the signals sent on the communication lines by precoding, andto estimate the coefficients of the crosstalk channels betweencommunication lines. The operator-side modems may be configured toadhere to the G.vector amendment to the VDSL2 standard, or to operateaccording to principles that have been proposed to the G.vectorcommittee.

Each one of the plurality of M CPEs 230 may include a number ofcommunication devices such as a DSL modem, a splitter, and a telephone.The DSL modem may adhere to the current VDSL2 standard. Such a modem isreferred to hereinafter as a legacy modem. Alternatively, the modem mayadhere to the G.vector amendment of the VDSL2 standard, or, it mayadhere to principles that have been proposed to the G.vector committee,such as: measuring error samples relative to sync symbols, and returningerror samples to the CO via an upstream feedback channel. Such a modemis referred to hereinafter as a G.vector modem. The integer M indicatesthe number of CPEs present in the communication system. There may alsobe other communication lines coming from the CO 210 that are notcontrolled by the VCE and are not part of the vectoring group 220, andare not depicted in FIG. 2.

The vectoring group 220 includes a plurality of M communication lines.Each one of the plurality of M communication lines may correspond and beconnected to one of the plurality of M CPEs 230. The plurality of Mcommunication lines may include a plurality of first communicationlines, and at least one second communication line. The firstcommunication lines and the at least one second communication line maybe different from each other. For example, if one of the plurality of MCPEs 230 includes a G.vector modem, the corresponding communication lineis referred to as a G.vector communication line. If one of the pluralityof M CPEs 230 includes a legacy modem, the corresponding communicationline is referred to as a legacy communication line. Example embodimentsof the present invention also include where each of the plurality of Mcommunication lines is the same type of communication line.

Referring to FIG. 2, the CO 210 is configured to transmit sync symbolsthat communicate pilot sequences and flag sequences according to exampleembodiments. The pilot sequences are used to estimate crosstalkcoefficients between the plurality of M communication lines, and theflag sequences are used to communicate sync-flags from the CO 210 to theplurality of M CPE modems.

FIG. 3 illustrates a modulation system that is part of CO 210 accordingto example embodiments. The CO 210 may include a first code modulator301, a second code modulator 302, a combiner 303, and a channelmodulator 304.

The first code modulator 301 is configured to receive a pilot sequence.The pilot sequence is a sequence of values; for example, the values mayinclude either “1” or “−1”. The length of the pilot sequence is L, whereL is an integer greater than or equal to 1. The first code modulator 301is also configured to receive a pilot code. The pilot code is a fixedsequence of length C, where each element of the pilot code includes acomplex number and/or at least two elements of the pilot code arenon-zero.

The first code modulator 301 is configured to modulate the pilotsequence by multiplying each value of the pilot sequence by a pilotcode. For instance, the first code modulator 301 forms a Kroneckerproduct of the pilot sequence with the pilot code. Thus, if the pilotsequence is of length L, the first code modulator 301 forms an encodedsequence of length CL. Each element of the original pilot sequence isreplaced in the encoded sequence of length CL by a sub-sequence oflength C obtained by multiplying each value of the pilot code by thegiven element of the original pilot sequence.

The second code modulator 302 is configured to receive a flag sequence.The flag sequence is also a sequence of values; for example, the valuesmay include either “1” or “−1”. The length of the flag sequence is L,where L is an integer greater than or equal to 1. The second codemodulator 302 is also configured to receive a flag code. Similar to thepilot code, the flag code is a fixed sequence of length C, where eachelement of the flag code includes a complex number and/or at least twonon-zero values.

The second code modulator 302 is configured to modulate the flagsequence by multiplying each value of the flag sequence by a flag code.Similar to the first code modulator 301, the second code modulator 302forms a Kronecker product of the flag sequence with the flag code. Thatis, if the flag sequence is of length L, the second code modulator 302forms an encoded sequence of length CL. Each element of the originalflag sequence is replaced in the encoded sequence of length CL by asub-sequence of length C obtained by multiplying each value of the flagcode by the given element of the flag sequence. The flag code and thepilot code are preferably mutually orthogonal to each other. Details ofthe pilot and flag code will be further described later.

The combiner 303 is configured to receive both the encoded pilotsequence and the encoded flag sequence and to add the encoded pilotsequence to the encoded flag sequence to obtain a resulting sync symbolsequence of length CL.

The channel modulator 304 is configured to transform each sync symbol ofthe resulting sync symbol sequence into a channel signal suitable fortransmission across a communication line within the plurality of Mcommunication lines. Then, the resulting channel signal sequence istransmitted to a corresponding CPE during a sequence of the syncperiods, with 256 data signals transmitted between each consecutive pairof sync symbols. This transformation may include subsystems commonlyused in DSL transmitters, such as scramblers, and Fouriertransformation.

FIG. 4 illustrates a system that is part of a CPE according to anexample embodiment. The CPE is configured to receive the transmittedsignal and recover both the pilot sequence and the flag sequence. TheCPE includes a channel demodulator 401, a first code demodulator 402, asecond code demodulator 403, a first estimation system 404, a secondestimation system 405, a combiner 406 and an error feedback unit 407.

The channel demodulator 401 is configured to receive a noisy version ofthe signal transmitted by the CO during a sync symbol period andgenerate a complex value that is a noisy received sync symbol. Thechannel demodulator 401 performs the inverse operation of the channelmodulator 304, and may include subsystems commonly used in DSLreceivers, such as a descrambler, a Fourier transform andfrequency-domain equalization. The noisy sync symbols produced by thechannel modulator 304 during CL consecutive sync symbol periods producea noisy sync symbol sequence of length CL. The noisy version of thetransmitted resulting sync symbol sequence is the transmitted resultingsync symbol sequence of length CL, corrupted by background noise as wellas by interference from signals transmitted on other communication linesduring sync symbol periods.

Referring to the lower branch, the second code demodulator 403 isconfigured to correlate the noisy sync symbol sequence with a flag codeto generate a noisy flag sequence. In an example embodiment, the flagcode used at the CPE in FIG. 4 is equal to or proportional to the flagcode used in FIG. 3, and is orthogonal to the pilot code used in FIG. 3.To perform the demodulation, the second code demodulator multiplies eachof C consecutive noisy sync symbols by the complex conjugate of acorresponding one of C flag code values. The C products resulting fromeach of these multiplications are summed to produce a single noisy flagvalue. In this way, a sequence of CL noisy sync symbols is demodulatedto produce a noisy flag sequence of length L. This operation will befurther explained with reference to FIG. 5.

The second estimation system 405 is configured to receive the noisy flagsequence and generate an estimated flag sequence. For each noisy flagvalue, the second estimation system 405 estimates which of two possibleflag values was originally sent by the CO 210. The second estimationsystem 405 may be a slicer, for example.

As a result, the channel demodulator 401, the second code demodulator403 and the second estimation system 405 depicted in FIG. 4 recovers theflag sequence from the transmitted resulting sync symbol sequence bygenerating an estimated flag sequence. In a system with multiple tones,the estimator may take into account the received values on multipletones.

Referring to the upper branch, the first code demodulator 402 isconfigured to correlate the noisy sync symbol sequence with the pilotcode to generate a noisy pilot sequence. In an example embodiment, thepilot code used at the CPE in FIG. 4 is equal to or proportional to thepilot code used in FIG. 3, and is orthogonal to the flag code used inFIG. 3. To perform the correlation, the first code demodulator 402multiplies each of C consecutive noisy sync symbols by the complexconjugate of a corresponding one of C pilot code values. The C productsresulting from each of these multiplications are summed to produce asingle noisy pilot value. In this way, a sequence of CL noisy syncsymbols is demodulated to produce a noisy pilot sequence of length L.This operation will be further explained with reference to FIG. 5.

The first estimation system 404 is configured to receive the noisy pilotsequence and generate an estimated pilot sequence. For each noisy pilotvalue, the first estimation system 404 estimates which of severalpossible pilot values was originally sent by the CO. For example, for abinary pilot sequence, the estimator may estimate whether the initialpilot value was “1” or “−1”. The first estimation system 404 may be aslicer, for example.

The combiner 406 is configured to subtract the estimated pilot valuefrom the noisy pilot sequence in order to obtain a pilot error sample,which in general consists of background noise and interference fromother communication lines.

The error feedback unit 407 is configured to transmit a sequence of thepilot error samples back to the CO 210, to be used in crosstalkestimation. In an alternative example embodiment, the pilot values ofthe pilot error sequence may be known a priori. In this case, the firstestimation system is not needed, and error samples are determined bysubtracting the known pilot value from the noisy pilot value.

Advantageously, only one error sample needs to be sent back for everygroup of C received sync symbols. The upstream bandwidth required tofeed back the error samples is therefore C times less than would berequired by a system that feeds back an error sample for each syncsymbol.

According to an example embodiment, the values of the flag code and thepilot code may include at least two non-zero values. In a particularexample embodiment of including at least two non-zero values, the lengthof the pilot code and the flag code may be C=2. The flag code tomodulate the flag sequence is of the form (a, a) and the pilot code tomodulate the pilot sequence is of the form (b,−b), where a=b=√{squareroot over (2)}/2. The flag code (a, a) and the pilot code (b,−b) areorthogonal to each other.

In this particular example embodiment, to calculate and send a resultingsync symbol sequence of a flag value “1” and a pilot value “−1” based onthe pilot and flag code (b,−b) and (a,a), the resulting sync symbolsequence is 1·(a,a)+(−1)·(b,−b)=(a−b, a+b). The pilot value is recoveredas (a−b,a+b)·(a,a)=2a²=1, and the sync value is recovered as(a−b,a+b)·(b,−b)=−2b²=−1.

According to another example embodiment, the values of the pilot andflag codes may be complex. For example, the complex values of the pilotand flag codes may be actual constellation points sent on VDSL2 syncsymbols such as “1+j” and “−1−j”. In this particular example embodiment,the flag code may be (a,a) to modulate the flag sequence, and the pilotcode may be (b,−b) to modulate the pilot sequence, where a and b are anycomplex constellation point. The flag code for demodulation might be(1,1) and the pilot code for demodulation might be (1,−1).

In this particular example embodiment, a and b may be as follows:

${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$where α²+β²=1, and where α and β are first and second parameters of theflag and pilot codes.

FIG. 5 illustrates a diagram depicting the transmitted resulting syncsymbol sequence, the recovered pilot sequence and flag sequenceaccording to example embodiments. Referring to FIG. 5, the sync symbols(e.g., the middle row) illustrate actual constellation points of theresulting sync symbol sequence. The consecutive pairs of the resultingsync symbol sequence can be added together to obtain the flag values,and the pilot values can be obtained from the difference between thesame pair of consecutive sync symbols. In this particular example, thefirst two values of the consecutive resulting sync symbol sequence areadded together to obtain the first value of the flag sequence. The firsttwo values of the consecutive resulting sync symbol sequence aresubtracted from each other to obtain the first value of the pilotsequence. In this particular example, the flag sequence is (+1,+1,+1,−1)and the pilot sequence is (+1,−1,−1,+1).

FIG. 6 illustrates the four different complex sync symbols that mayarise according to example embodiments, assuming that the pilot sequenceand the flag sequence are bipolar, consisting only of values 1 or −1.Under this assumption, the sync symbol may include four possible values:a+b, a−b, −a−b, and −a+b. When a and b are the complex points specifiedabove, each sync symbol has a unit magnitude.

Referring back to FIG. 2, the CO 210 is configured to allocate syncsymbol power between the pilot sequence and the flag sequence byadjusting the first and second parameter α and β of the flag and pilotcodes. By adjusting the first and second parameters α and β, sync symbolpower can be allocated between the pilot sequence and the flag sequence.On the left side of FIG. 6, the first parameter is greater than thesecond parameter (i.e., α>β). As a result, more power is allocated tothe flag sequence. On the right side of FIG. 6, the first parameter isless than the second parameter (i.e., α<β). As a result, more power isallocated to the pilot sequence.

In the conventional time-sharing solution, exactly half of the power isused for sending flag sequences, and half is for sending pilotsequences. According to example embodiments, the power allocated to flagsequences may be reduced and more power may be allocated to pilotsequences. The increased power to pilot sequences would allow thecrosstalk estimation to take place more quickly. Alternatively, whengood crosstalk estimates have already been obtained, the power could beshifted to the flag sequences for increased reliability of sync-flags.

According to example embodiments, power-sharing between flag sequencesand pilot sequences is particularly advantageous when a communicationline is terminated by a legacy modem. For example, the CO 210 maytransmit sync symbols to the legacy modem such that much less power isallocated to a pilot sequence than to a flag sequence. Advantageously,the power assigned to the pilot sequence may be chosen to be smallenough so that the ability of the legacy modem to detect sync-flagsaccording to traditional techniques is not substantially reduced. At thesame time, the power of the pilot sequence may be chosen to be largeenough to influence error measurements on a second communication line,so that it is possible for the VCE to estimate crosstalk from the legacyline into the second line using correlation.

The communication system according to example embodiments is depictedfor one tone of one communication line. The operation described above issimilarly performed in parallel on other tones and on othercommunication lines. Example embodiments of the present invention mayalso be applied in embodiments where some tones and some lines onlyapply part of the elements depicted in FIGS. 3 and 4. For example, thecommunication system associated with one given tone and communicationline may only send a pilot sequence, the communication system associatedwith another tone and/or communication line may only send a flagsequence, and the communication system associated with another toneand/or communication line sends both a flag sequence and a pilotsequence. Likewise, at the receiver (e.g., CPE), only flag values may beestimated for some tones and communication lines, only pilot errorvalues may be estimated for some tones and/or communication lines, andboth flag values and pilot error values may be estimated for some tonesand/or communication lines.

Note that even when one communication line only transmits encoded flagsequences and another line only transmits encoded pilot sequences,crosstalk between lines may often cause a received signal to consist ofa sum of an encoded flag sequence and encoded pilot sequence. Thus, thesystem depicted in FIG. 4 is still needed to separate flag sequencesfrom code sequences even in this case. For example, a CO 210 may use thelower branch depicted in FIG. 3 to send a flag sequence on a firstcommunication line, and simultaneously use the upper branch depicted inFIG. 3 to send a pilot sequence on a second communication line. Due tocrosstalk, the received signal at the CPE of the first communicationline would include a sum of the signals sent on the first and secondcommunication lines. The CPE of the first communication line may use thelower branch system of FIG. 4 to estimate the flag sequence sent on thefirst communication line. The pilot sequence sent on the second linewould appear as part of the pilot error sequence computed by the upperbranch of the system of FIG. 4. Note that, in this case, if the CPEknows that no pilot sequence was sent on the first communication line,then the first estimation system 404 is not needed since the pilotsequence is known to be zero.

Although the illustrated example embodiments used code sequences oflength two, example embodiments also encompass embodiments with codesequences of length C>2. In such example embodiments, the code sequencesmay be orthogonal code sequences of length C. A subset of code sequencesmay be used to modulate pilot values, and another subset of codesequences may be used to modulate flag values.

The forgoing discussion has focused on downstream transmission of pilotand flag signals from a central office to CPE modems, and on downstreamsystem in which crosstalk is cancelled by precoding. However, thepresent invention also can be applied to systems in which pilot and flagsignals are sent upstream from CPE modems to a central office, and wherecrosstalk is cancelled by coordinated signal processing at the centraloffice (e.g. by “postcoding”).

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the invention, and all such modifications areintended to be included within the scope of the invention.

We claim:
 1. A method for sharing sync symbols to communicate flagsequences and pilot sequences during a sequence of sync symbol periods,the method comprising: modulating the pilot sequence by multiplying apilot code by each value of the pilot sequence to form an encoded pilotsequence; modulating the flag sequence by multiplying a flag code byeach value of the flag sequence to form an encoded flag sequence, thepilot and flag codes being mutually orthogonal to each other, the pilotand flag codes each including at least two non-zero values; generating aresulting sync symbol sequence based on the encoded pilot sequence andencoded flag sequence such that each value of the encoded pilot sequenceand each value of the encoded flag sequence affects more than one syncsymbol in the resulting sync symbol sequence; and sending the resultingsync symbol sequence during the sequence of sync symbol periods.
 2. Themethod of claim 1, wherein sending the resulting sync symbol sequenceincludes transforming each sync symbol of the resulting sync symbolsequence into a signal for transmission across a communication line. 3.The method of claim 1, wherein generating the resulting sync symbolsequence includes adding the modulated pilot sequence and the modulatedflag sequence.
 4. The method of claim 1, wherein the values of the flagand pilot codes are complex numbers.
 5. The method of claim 1, whereineach value of the pilot sequence and the flag sequence is one of tworeal or complex values.
 6. The method of claim 1, further comprising:allocating sync symbol power between the pilot sequence and the flagsequence by adjusting a first and second parameters of the pilot andflag codes.
 7. The method of claim 6, wherein the flag code is (a, a)and the pilot code is (b, −b), wherein${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$α²+β²=1, and wherein α and β are the first and second parameters of theflag and pilot codes.
 8. The method of claim 1, wherein the pilotsequence is used for crosstalk estimation and flag sequence is used tosend sync flags.
 9. The method of claim 1, wherein the resulting syncsymbol sequence includes at least one pair of sync symbols, a sum of theat least one pair of sync symbols being a flag value of the flagsequence, and a difference between the at least one pair of sync symbolsbeing a pilot value of the pilot sequence.
 10. A communication systemfor sharing sync symbols to communicate flag sequences and pilotsequences during a sequence of sync symbol periods, the communicationsystem comprising: a first modulator configured to modulate the pilotsequence by multiplying a pilot code by each value of the pilot sequenceto form an encoded pilot sequence; a second code modulator configured tomodulate the flag sequence by multiplying a flag code by each value ofthe flag sequence to form an encoded flag sequence, the pilot and flagcodes being mutually orthogonal to each other, the pilot and flag codesincluding at least two non-zero values; and a combiner configured tocombine the encoded pilot sequence with the encoded flag sequence togenerate a resulting sync symbol sequence such that each value of theencoded pilot sequence and each value of the encoded flag sequenceaffects more than one sync symbol in the resulting sync symbol sequence.11. The communication system of claim 10, further comprising: a channelmodulator configured to send the resulting sync symbol sequence bytransforming each sync symbol of the resulting sync symbol sequence intoa signal for transmission across a communication line.
 12. Thecommunication system of claim 10, wherein the communication system isconfigured to allocate sync symbol power between the pilot sequence andthe flag sequence by adjusting a first and second parameter of the pilotand flag codes.
 13. The communication system of claim 12, wherein theflag code is (a, a) and the pilot code is (b, −b), wherein${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$wherein α²+β²=1, and wherein α and β are the first and second parametersof the flag and pilot codes.
 14. The communication system of claim 10,wherein the pilot sequence is used for crosstalk estimation and flagsequence is used to send sync flags.
 15. The communication system ofclaim 10, wherein the values of the flag and pilot codes are complexnumbers.
 16. The communication system of claim 10, wherein each value ofthe pilot sequence and flag sequence is one of two real or complexvalues.
 17. The method of claim 10, wherein the resulting sync symbolsequence includes at least one pair of sync symbols, a sum of the atleast one pair of sync symbols being a flag value of the flag sequence,and a difference between the at least one pair of sync symbols being apilot value of the pilot sequence.
 18. A method for sharing sync symbolsto communicate flag sequences and pilot sequences during a sequence ofsync symbol periods, the method comprising: modulating the pilotsequence by multiplying a pilot code by each value of the pilotsequence; modulating the flag sequence by multiplying a flag code byeach value of the flag sequence, the pilot and flag codes being mutuallyorthogonal to each other, the pilot and flag codes each including atleast two non-zero values; generating a resulting sync symbol sequencebased on the modulated pilot sequence and modulated flag sequence; andsending the resulting sync symbol sequence during the sequence of syncsymbol periods, wherein the flag code is (a, a) and the pilot code is(b, −b), wherein${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$wherein α²+β²=1, and wherein α and β are the first and second parametersof the flag and pilot codes.
 19. A communication system for sharing syncsymbols to communicate flag sequences and pilot sequences during asequence of sync symbol periods, the communication system comprising: afirst modulator configured to modulate the pilot sequence by multiplyinga pilot code by each value of the pilot sequence; a second codemodulator configured to modulate the flag sequence by multiplying a flagcode by each value of the flag sequence, the pilot and flag codes beingmutually orthogonal to each other, the pilot and flag codes including atleast two non-zero values; and a combiner configured to combine themodulated pilot sequence with the modulated flag sequence to generate aresulting sync symbol sequence, wherein the flag code is (a, a) and thepilot code is (b, −b), wherein${a = {\frac{\alpha}{\sqrt{2}}( {1 + j} )}},{b = {\frac{\beta}{\sqrt{2}}( {1 - j} )}},$wherein α²+β²=1, and wherein α and β are the first and second parametersof the flag and pilot codes.